Telephone system



Feb. 25, 1936. F A BONOM] 2,031,922

TELEPHONE SYSTEM Filed July 29, 1932. 5 Sheets-Sheet l n r I INVENTOR F: A. BONOM/ ATTORNEY F. A. BONOMI TELEPHONE SYSTEM Filed July 29, 1932 5 Sheets-Sheet 2 A A TTORNE V W mm W8 WA TELEPHONE SYSTEM Filed July 29, 1952 s Sheets-$11633 FIG. 3

INVE/VTOR E A. BONOM/ ATTORNEY Patented Feb. 25, 1935 iitiTEE g'l'ATEh PATENT GFFICE Bell Telephone Laboratories,

Incorporated,

New Ycrk, N. Y., a corporation of New York Application July 29, 1932, Serial No. 625,561

11 Claims.

This invention relates to telephone systems and more particularly to improvements in private branch exchange systems.

In private branch exchange systems that are in common use today, a plurality of subscribers lines, usually located in one building such as an apartment house, a large business establishment or the like, terminate at a local switchboard located in the building and operated by an attendant. Trunk line's incoming from central offices also terminate at the P. B. X. switchboard so that, through the intermediation of the attendant, calls to and from the P. B. X. subscribers lines may be completed at the P. B. X. board. That is, all calls originating at the subscribers station as well as those incoming to or intended for the subscriber are completed by the P. B. X. attendant at the P. B. X board.

Such systems have given satisfactory service in the past but with the ever increasing size of our modern apartment houses and the class of people occupying them it has been found necessary not only to furnish the subscribers lines with P. B. X. service in such cases but to also provide direct central office service so as to modify the duties of the attendant from those of an ordinary telephone operator to those of a secretary.

It is accordingly the object of this invention to improve upon existing private branch exchange systems so as to combine direct central ofiice telephone service with that ordinarily furnished by the private branch exchange switchboard.

This object is attained in accordance with a feature of the invention by the provision of means whereby central ofiice calls may be received and originated at a station, having access to P. B. X. service, independently of the P. B. X. attendant.

Another feature of the invention resides in the provision of means, under the control of the P. B. X. subscriber, for the interception by the P. B. X. attendant of all incoming central ofiice calls to the subscribers lines.

A further feature of the invention provides for the exclusive use of the central ofiice line, normally associated with the subscribers line, by the subscriber. By this feature, the P. B. X. operator is prevented from originating central oflice calls over the line allotted to the subscribers line when the latter is conditioned for intercepting service.

A still further feature of the invention resides in the provision, at the subscribers station, of separate distinctive toned ringers whereby the subscriber is enabled to distinguish between incoming central ofiice calls and. calls completed to his station at the P. B. X. board.

Another feature of the invention is embodied in a particular chain arrangement of the talk keys associated with the P. B. X. operator's cord circuits whereby the operator is prevented from listening in on more than one connection at a time.

These and other features not specifically identified, will appear from the following detailed description made with reference to the accompanying drawings in which:

Fig. 1 shows two P. B. X. lines terminating at a private branch exchange and the rear ends of a 15 regular cord circuit and an intercepting cord circuit employed in setting up connections from and completing connections to stations on the lines;

Fig. 2 shows the major portions of the attendants cord circuits and dialing equipment and in reading the drawings Fig. 2 is placed to the right of Fig. 1;

Fig. 3 shows an intercepting trunk with which is associated a subscribers line, the trunk circuit interconnecting a private branch exchange and a central ofiice;

Fig. 4 shows a two-way trunk terminating at the P. B. X.

Special reference is now made to Fig. 3 in describing a condition in which the interception of calls intended for a P. B. X. subscriber by a P. B. X. attendant is under the control of the attendant at the P. B. X. board.

Assuming the subscriber at station A is at home and desires calls intended for him to come 5 direct to his station and not be intercepted by the attendant, a dummy plug generally indicated at It] would be inserted in jack II of the intercepting trunk circuit I2. Under this condition, ringing current emanating from the central ofiice traverses trunk conductor I3, upper winding of relay I4, conductor I5, conductor I6, ringer I'I, conductor I8, lower winding of relay I I, trunk conductor I9 to the ringing current source at the central ofiice. The ringer I'l operates in this circuit to signal the subscriber at station A. In response to this signal the subscriber removes the receiver at station A, whereupon relay I 4 operates from central oflice battery over a circuit including trunk conductor I3, upper winding of 5 relay I4, conductor I5, lower normal contact of dial 213, conductor 2|, transmitter 22, receiver switch arm 23, conductor I8, lower winding of relay it, trunk conductor I9 to the central office battery. Relay I4 in operating causes relay 24 to operate in an obvious circuit. Relay 24 performs no useful function at this time because the line is not conditioned for intercepting service.

When a subscriber desires his incoming calls to be intercepted by the P. B. X. attendant for any reason, the dummy plug as is removed from jack II. This operation closes jack contact 25, which connects the upper and lower windings of relay 23 in series with the tip and ring condensers 2! and 28 across the tip and ring of the trunk [2.

When ringing current is applied to the trunk circuit l2 at the central ofiice, the subscribers bell H at station A operates as before and relay 26 operates as long as ringing continues. Relay 26 operated, connects the upper winding of relay 29 to the mid-point of the ringing bridge. Relay 29 operates to ground in multiple with the upper winding of relay 2% and condenser 2?. Relay 29 in operating locks in a circuit extending from grounded battery, front contact and lower armature of relay 29, upper armature and front contact of relay 29, conductor 30, back contact and outer lower armature of relay 3|, conductor 32 to .ground through its own lower winding. Relay 29 also causes lamp '32 to be lighted in a circuit extending from grounded battery, front contact and lower armature of relay 29, conductor 33, normal make-before-break contact 3 3 of relay 3 I, conductor 35 to ground through the filament of lamp 32. Relay 29 also connects battery to one winding terminal of relay 3| by way of conductor 33.

The combination of relays 26 and 29 and the method of connecting relay 29 from the mid-point of the ringing bridge, prevents the lamp 32 from lighting falsely when the plug of an intercepting cord is removed from jack II and also prevents the lamp 32 from lighting falsely in case the subscriber at station A dials when his line is conditioned for intercepting service.

Upon noting the lighted lamp 32, the P. B. X.

attendant inserts plug 36 of intercepting cord" 3'! into jack H associated with the lighted lamp 32, which operation opens up the ringing bridge at jack contact 25 and causes relay 31 to operate in a circuit extending from grounded battery, front contact and lower armature of relay 29, conductor '33, winding of relay 3|, lower normal make-before-break contact 38 of relay 3|, and its armature, ring contacts of jack II and plug 36, armature and back contact of relay 39, resistance element 49 to ground.

Relay 3! operated, locks in a circuit extending from grounded battery, front contact and lower armature of relay 29, conductor 33, winding and contact M of relay 3|, sleeve contacts of jack M and plug 36 to ground through the winding of relay 39. Relay 3|, at its innermost front contacts-and armatures connects the tip and ring conductors l3 and IQ of the intercepting trunk 12 through to the P. B. X. attendants cord circuit 37. At its contact 34, it opens the circuit to the lamp '32 and at its lower outer armature and front contact it opens the locking circuit for relay 29, which releases.

Relay 3! locks in series with relay 39 in the attendants intercepting cord circuit 31, relay 39 operating in this circuit. Relay 39 operated, disconnects ground from the ring contact of plug '36, connects the ring conductor through to a contact of relay H and connects relay 12 across the tip and ring of the cord 31.

The attendant now actuates the talk key 13, whereupon relay ll operates in a circuit extending from grounded battery, winding of relay H, alternate contacts of key '83, conductor 13, normally closed contacts of key l5 which is the last key in the chain of keys. associated with the attendants regular cords, normally closed contacts of key 16, conductor H to ground on the normally closed contact of key 78 which is the first key in the chain. Relay H operated, connects ground to the attendants transmitter by way of the front contact and outermost upper armature of relay H and conductor 19. At its outermost lower armature and middle upper armature relay ll connects the tip and ring of the cord 3"! to corresponding conductors of the attendants telephone circuit AT, and at its inner low-er armature, it connects the secondary winding of relay 72 across the tip and ring of the cord by way of the normally closed upper makebefore-break contacts of relay 8!].

Relay 12 may or may not operate on ringing current but it will operate on direct current supplied during the silent interval. Relay i2 oper ated, causes relays 8!, 82 and 83 in the attendants telephone circuit to maintain the secondary winding of relay 12 across the cord long enough to trip machine ringing first and then to connect the attendants telephone set across the tip and ring of the cord.

The attendant, after receiving the necessar; instructions about the call and imparting the called station instructions, releases key l3 and removes plug 36 from Jack H releasing all the relays operated and returning the circuit to normal.

When, a subscriber whose line, such as the line associated with station A, has been conditioned for intercepting service by the insertion of a dummy plug l9 into the jack ll, removes his receiver from the switchhook in response to an incoming call, relay it operates as hereinbefore described over the central oiiice loop, which causes relay 24 to operate in an obvious circuit. Relay 24 in operating disconnects the tip and ring conductors I3 and t9 from the contacts of relay 3! The P. B. X. attendant is thereby prevented from associating her telephone circuit with a line after the subscriber has answered, and in case the subscriber removes his receiver after a call has been intercepted by the P. B. X.

' attendant. the attendant is immediately cut off by the operation of relay 24.

It will be noted that the trunk circuit l2 which is individual to the subscribers station A is not accessible to the P. B. X. attendant for the purpose of originating outgoing central oflice calls.

This is rendered possible by the fact that relay 3|, which completes the connection of the tip and ring conductors of the trunk i2 with the attendants cord, can not be operated unless relay 29 is operated which operates when relay 26 operates, the latter being operable from the central'office end only.

The next description is directed to intercepting service in which the subscriber is provided with the necessary equipment at his station to elect whether or not incoming calls are to be routed to the P. B. X. attendant.

InzFig. 1 the line 53 incoming from a central office is shown normally associated with contacts 5-! and 52 of key 53 which is located at the subscribers station E and is furnished as part of the substation equipment.

Ringing current incoming over the line 53 operates the ringer 5 5. The station B is furnished with two ringers 54 and 55 each being differently pitched or furnishing a distinctive signal so that the subscriber at station B may determine whether or not the incoming call originated at a central office or at the local P. B. X. as will appear presently.

In response to the signal 54, the subscriber at station B removes his receiver to answer the call in the usual manner.

When a subscriber desires his incoming calls to be intercepted by the P. B. X. attendant for any reason, the key 53, which is a locking key, is operated. The operation of the key 53 disconnects the substation B from the central oflice line 59 at contacts 5| and 52 and substitutes therefor by way of contacts 55 and 59, the intercepting trunk 51, which terminates at the P. B. X. attendants position in jack 58.

When ringing current is applied to the line 50 at the central office, the ringer 54 is rung at station B and relay 59 operates by way of its'lower winding. Relay 59 operated, establishes a locking circuit for itself extending from grounded battery, lower armature and front contact of relay 59, front contact and upper armature of relay 59, back contact and outermost upper armature of relay 69, to ground by way of the upper winding of relay 59. It also prepares an operating circuitfor relay 69 and establishes a circuit for lamp 6| by way of the upper makebefore-break contacts of relay 56.

The P. B. X. attendant answers the incoming call in response to the'lighted lamp 8| by inserting the plug of an intercepting cord 31 into jack 58. Relay 69 thereupon operates over the ring circuit including resistance element and contacts 62 of relay 69 and immediately locks over the sleeve circuit in series with relay 39. Relay 98 operated, closes the tip and ring conductors of the intercepting trunk 51 through to the P. B. X. attendant, opens the energizing circuit for lamp BI and opens the locking path for relay 59 which releases.

After the completion of the call, the P. B. X. attendant removes plug 36 from jack 58 causing relay 69 to release and restore the circuit to normal.

Should the key 53 at station B be actuated for intercepting service and the subscriber desire to answer a call incoming over the line in response to the operation of ringer 56, it is necessary that he restore the key to normal before removing his receiver from its switchhook. Failure to do this would result in the operation of line relay 63 and the consequent lighting of lamp 64 at the P. B. X. attendants position. Under this condition, that is, with key 53 operated and the receiver at station B ofi the hook, the relay 93 would operate in a circuit extending from grounded battery, normal ring contacts of jack 66, conductor 65, contacts 61 of key 53, conductor 68, through the closed subscribers station loop, conductor 69, contacts III of key 53, conductor II to ground at the normal tip contacts of jack 66.

With the key 53 operated, it is impossible for the P. B. X. attendant to originate central ofiice calls over the line 59 since it is impossible to operate relay 69 to close the trunk circuit 51 through to the attendant, except when relay 59 is operated which can only be operated from the central ofiice end.

As hereinbefore stated, the ringers 54 and located at station B are separate, distinctively toned ringers in order to indicate to the subscriberwhether a local or central office call is incoming, the former by way of a line circuit made up of conductors and 1| and the latter by way of a central oflice line circuit such as 59. It will be noted that the intercepting trunks 51 and I2 require ground potential on the ring lead in order to operate relays 99 and 3!, respectively, to connect the attendant through to the line. It is, therefore, impossible for the attendant to employ a regular cord circuit to handle calls on these trunks, because the attendants regular cords furnish battery on the ring lead as will appear in a subsequent description. The reason it is undesirable to allow the attendant to employ a regular cord in handling intercepted calls is that the subscriber is charged for the exclusive use of the intercepting trunk associated with his line and should therefore be guaranteed that no other party can originate central office calls over the trunk. By furnishing battery potential on the ring lead of the attendants regular cords and requiring ground potential on the ring lead of the trunks to operate relays 99 and 3|, the attendant must of necessity employ an intercepting cord 3'! when handling a call on an intercepting trunk, the cord 31 being capable of providing the necessary ring lead potential.

In order to furnish a complete understanding of the service possible with the system herein disclosed, it will now be shown how a connection between an unrestricted service station C and a restricted service station B is effected.

A subscriber at the unrestricted service station C removes his receiver from its switchhook estabe lishing an obvious circuit for the line relay 88 which operates causing the line lamp 89 to be lighted as a signal to the attendant. The attendant thereupon inserts plug of her regular cord circuit AC into jack 99 associated with the lighted line lamp 89 thereby opening the energizing circuit for relay 89 and extinguishing the line lamp 89. With plug 85 in jack 9!), relay 86 operates in series with the winding of relay 81 in an obvious sleeve-circuit. Relay 9! is a marginal relay and does not operate owing to the high resistance in the sleeve of jack 90.

Relay 86 operated, operates relay 9| in an obvious circuit. Relay9| at its upper inner armature disconnects the busy test conductor 92 from the tip of the rear cord. At the outer upper armature relay 9| establishes a circuit for relay 93 which may be traced from grounded battery, winding of relay 93, conductor 94, outer upper armature and front contact of relay 9|, conductor 95, outer upper armature and back contact of relay 96, conductor 97 to ground on a normal contact of key 98. At its two inner armatures relay 9| connects relays 99 and I98 to the tip and ring leads of the rear cord whereupon these relays operate. The circuit for relays 99 and I may be traced from grounded battery, lower winding of relay I99, outer lower armature and back contact of relay IUI, conductor I92, a normally closed contact of key 98, conductor I93, lower winding of relay 99, a normally closed contact of key I04, conductor I95, back contact and lower armature of relay I05, front contact and inner lower armature of relay 9|, ring contacts of plug 85 and jack 99, through the closed subscribers loop at station C, tip contacts of jack 98 and plug 85, inner upper armature and front contact of relay 9|, upper armature and back contact of relay I95, conductor I 91, a normally closed contact of key I94, upper winding of relay 99, conductor I98, a normally closed contact of key 99, conductor I99, outer upper armature and,

back :contact of relay IOI, conductor 0., upper winding of relay I00, conductor 91, to ground at a normally closed contact of key .98. Relays '99 and I operate in this circuit. Relay E93 "operated, performs no useful function at this time. Relay 99 operated prevents the rear cord supervisory lamp III from lighting while relay 1.00 short circuits the retardation .0011 .I.I2.

The attendant now actuates the Talk and Dial key 18 whereupon relay I13 operates in an obvious circuit and causes relay II I to operate in an obvious circuit. Relay .I I3, at .its lower outer armature and'front contact connects ground to the attendants transmitter circuit by way of conductor .5. Relay H3 at its inner armatures and front contacts and relay H4 at its outer armatures and .front contacts connect-the .attendants telephone set across the tip :and ring of the rear cord.

The attendant now receives the required information from the unrestricted service station C, releases the Talk and Dial key 18, releasing'relays I I3 and I I4 and inserts the front cord plug'rl I 0 into the restricted service line jack 60, causing relays I-I1 and I I8 to operate in series in an obvious sleevecircuit. Relay II8 causes relay 96 to operate in an obvious circuit while relay I I1 maintains an obvious operating circuit for relay 93. Relay '93 being 'keptoperated, prevents relay IOI from operating, preventsa short circuit from being placed across the tip and ring leads of the rear cord when relay 96 operates and causes the front supervisorylamp il 20 :to light under control of relays I2I and '96. The circuit for lamp I20 may be traced from grounded battery, filament of lamp I20, conductor I 22, inner upper armature and front contact of relay 93, conductor I23, lower armature and back contact of relay I2I., conductor [24, conductor I25, front contact and outer upper armature of relay '96, conductor 95, front contact and upper outer armature of relay 9-I, conductor 94, to ground at the armature and front contact of marginal relay I I1.

Relay 96 operated, disconnects the busy test lead from the tip of the front cord and connects the tip through to the front ringing key I21.

The attendant now rings the restricted service station E by operating key I21. The path for the ringing current extends from grounded ringing current source I28, lower alternate contact of key I21, ring contacts of plug H6 and jack 66, through the signaling circuit at station B, tip contacts of jack 66 and plug H6, inner upper armature and front contact of relay .96, to ground at the upper alternate contact of key I21. It will be noted that on a call completed at the P. 'B. X. the ringer 55 at stationB is actuated whereas, as hereinbefore described, on a call incoming from a central office to station B, the ringer 54 is actuated. The subscriber at station'B can readily determine the character of the incoming call since the ringers 54 and 55 are distinctively toned.

When the receiver at the station B is removed from the switchhook, relay I2I in the attendants cord operates causing the supervisory lamp I20 to be extinguished.

Under the conditions of the established connection between stations C and B by way of the cord circuit AC, battery and ground for the unrestricted service station C are furnished by the cord circuit AC through windings of relays I00 and 99; battery and ground for the restricted service station B are furnished by the cord circuit AC through windings of relay I21; the front austere and :rear cords of cord circuit AC .are coupled through condensers I30 and. I3I and no bridge is connected across the tip and ring of the cord except ringing relay I32 which performs no useful function at this time.

Both stations B and C have flashing and disconnect switchhook supervision.

In the case of a restricted service station such as station B calling another restricted service station, the call is usually answered by the attendant with'the front cord and completed with the rear cord. The relays in the cord operate in the same manner as just described in connection witha call originating at an unrestricted service station and intended for a restricted service station.

A call from an unrestricted service station for another restricted service station may be answered with either the front or rear cord and completed with either cord.

As it was necessary in the descriptions'so far completed to refer to certain relays such as '80, 8! and 82 in the attendants telephone and dialing circuit, it was deemed advisable to show this circuit in its entirety in order to render the disclosure fully understandable. To set forth the operations of this circuit, the following description is directed to a connection between an unrestricted subscribers station C and a central oflice with the attendant dialing.

As hereinbefore set forth, a call originating at an unrestricted subscribers station C is answered by the attendant with the rear cord of the cord AC. As all the operations incident to the insertion of plug 85 of the rear cord in jack 90 of the unrestricted subscribers line have already been described, it is deemed unnecessary to repeat them here.

The attendant upon learning that a central office call is desired by the subscriber at station and condenser I54 from across the tip and ring of the trunk I90, closes the tipand ring of the trunk from the cord AC to the central .office and transfers the holding path for relay II from the ring to the sleeveof the-cord. A circuit may now be traced from grounded battery, windings of relays I11 and H8, sleeve contacts of plug .6 and jack I59, lower make-before-break contacts of relay I52 to ground through the winding of relay I5I. Relay I 48 operates in this circuit, but relay I I1 being marginal does not operate owing to the high resistance, of the sleeve circuit just traced. Relay II8 causes relay 9'5 to operate to perform all the functions hereinbefore set forth in the description directed to the establishment of a connection between unrestricted and restricted subscribers lines. It will-be noted now that relay I I1 not having operated does not maintain a circuit for relay 93 which releases as a consequence of the operation of relay 96. Relay 93, released, causes relay I2I to operate 'in a circuit extending from grounded battery, lower Winding of relay I2I, conductor I56, lower makebefore-break contacts of relay 96, outer lower armature and back contacts of relay 93, conducand I16.

ill

tor I 51 to ground at the front contact and armature of relay 99.

Relay IOI operates in a circuit extending from grounded battery, winding of relay IOI, conductor I60, outer upper back contact and armature of relay 93, upper make-before-break contacts of relay 96, conductor 91, to ground at key 98. Relay 10! operated disconnects the condensers I30 and HM from the talking circuit, disconnects battery and ground through the windings of relay I from the ring and tip of the rear cord causing relay I96 to release. Relay I OI also disco-nnects battery and ground through the windings of relay I2I from the ring and tip of the front cord and connects the tip and ring of the front cord directly to the tip and ring of the rear cord respectively.

With the talk and dial key actuated and relays II3 and I14 operated as a consequence thereof, the attendant may now proceed to dial the desired central office number. When the attendant moves the dial I63 off-normal relay I64 operates by way of its upper winding to ground on the off-normal contacts of the dial. At its outer upper and lower contacts, relay I64 opens the attendants telephone circuit; at its upper inner contacts it establishes an obvious circuit for relay I65; at its lower front contact relay I64 connects resistance I66 across the leads I61 and E66. Relay I65 operated short circuits resistance I66 and establishes obvious circuits for relays I69 Relay I10 at its inner armatures and back contacts splits the cord circuit and connects battery and ground through the windings of relay Hi to the rear cord by way of conductors I12 and I13, leaving the dialing contacts of dial I63 connected to the front cord by way of conductors I61 and I68. Relay I10 operated, also connects retardation coil I14 across the leads I61 and I66 through the pulsing contacts of dial 563, the resistance 166 and retardation coil I14, however, being short-circuited by relay I65.

Relay I69 operated, provides an obvious looking circuit for relay I66. At its upper contact it disconnects the tip lead from the attendants telephone set and short-circuits the attendants head receiver to prevent an undesirable click when the dial returns to normal and to prevent the attendant from hearing the dial pulses.

Relay I15 operates in response to the operation of relay I10 and performs no useful functionat this time. Its primary function is to maintain the attendants receiver circuit normally closed. A circuit may now be traced from grounded battery, lower winding of relay I1 I inner lower front contact and armature of relay I10, conductor I12, inner lower armature and front contact of relay H3, inner lower armature and front contact of relay IOI, normal contacts of key 98, conductor I03, lower winding of relay 99, normal contacts of key I64, conductor I05, normal contacts of relay I06, inner lower front contact and armature of relay 9!, ring contacts of plug 85 and jack 90, over the closed subscribers line hook at station C, tip contacts of jack 90 and plug 85, upper inner armature and front contact of relay 9|, upper armature and back contact of relay I06, conductor I01, normal contacts of key I 04, upper winding of relay 99, conductor I06 normal contacts of key 98, conductor I09, inner upper front contact and armature of relay IOI, inner upper front contact and armature of relay II3, conductor I13, inner upper armature and front contact of relay I10, to ground through the upper winding of relay I1I. Relay I1I operated, closes an obvious locking circuit for relay I10.

As hereinbefore described, the dialing contacts are connected to the front end of cord, AC by way of conductors I61 and I68 so that the dial pulses are sent over the front cord and thence over thetrunk COT in the well known manner.

When the dial has completely restored, relay I65 releases removing the short circuit from resistance I66 and retardation coil I14 and also causes relay I69 to release. Relay I69 released, removes the short circuit from the attendants receiver and causes relay I 64 to release. The release of relay I64 removes the resistance I66 shunt from the retardation coil I14 and closes the attendants talking circuit. The retardation coil I now holds the central ofiice connection and the front and rear cords of cord A0 are connected through the condensers I11 and I18 to enable the station to talk on the connection even though the attendant should fail to restore the talk and dial key after dialing. When the key 15 is restored relays II 3 and IIQ-restore causing relays HI and I10 to release. Relay I10 released, causes relay I15 to release. Relay I15 is made slow to release in order to keep the attendants receiver circuit open long enough to prevent clicks in the receiver due to the release of key 18 in the cord circuit AC.

At the termination of the connection, the subscriber at station 0 replaces the receiver on its switchhook thereby causing relay 99 to release which in turn causes the release of relay I2 I. Relay 99 in releasing establishes an obvious circuit for supervisory lamp III which lights as a disconnect signal. Upon receipt of the disconnect signal the attendant may withdraw the front and rear cord plugs H6 and 85 from jacks I50 and 90 respectively whereupon all operated relays release and the circuit is restored to normal.

In previous descriptions, it has been shown that the operation of such keys as key 13 and I9l, which are individual to intercepting cord circuits such as 31, result in the connection of the attendants receiver to the intercepting cords by virtue of the operation of relay 1I. It has also been shown that the operation of keys such as 18, 16 and 15, which are individual to the attendants regular cord circuits such as AC, cause relays I I3 and II to operate to effect the association of the attendants receiver with the regular cords. Keys 13 and I9I of the intercepting cords and keys 18, 16 and 15 are all connected in a chain circuit, key 18 constituting the first key in the chain and furnishing ground potential for all the other keys. This arrangement precludes the possibility of an attendant listening in on more than one connection at a time. The operation of key 18 removes ground potential from all the other keys so that should an attendant actuate another key, such as 16 or 13, for the purpose of listening in on another connection, relays H3 and H4 of the regular cord with which key 16 is associated would not operate, nor would relay 1! of the intercepting cord with which key 13 is associated function. Should a key such as 15 be in its operated position and the attendants receiver therefor associated with the cord circuit to which key 15 is individual and any other key, such as key 16 or 18, having a lower position in the chain be actuated, the attendants receiver would be automatically disconnected from the cord corresponding to key 15 and connected to the cord corresponding to key 16 or 18 depending on which was operated.

What is claimed is:

1. In a telephone system, a subscribers station, asignaling device thereat, a link circuit individual to said subscribers station and terminating at a central office, an intermediate ofiice, a signaling device thereat, and means at said subscribers station for-disconnecting said station from said link circuit and causing the signaling device at said intermediate oflice to be actuated simultaneously with the signaling device at said station in response to a call incoming from said central oifice over said link circuit.

2. In a telephone system, a subscribers line circuit, a central ofiice, an intermediate ofiice, a

telephone line circuit normally connecting said.

subscribers line circuit with said central office, in intercepting trunk circuit terminating at said intermediate office, means under the control of the subscriber on said subscribers line circuit for transferring the connection of said telephone line circuit from said subscribers line circuit to said trunk circuit for the reception, at said intermediate'oifice', of calls incoming over said telephone line circuit, and means included in said intercepting trunk circuit for rendering. said telephone line circuit inaccessible at said intermediate office for theinitiation of calls to said central ofiice by way of said telephone line circuit.

3. In a. telephone system, a subscribers station, a central ofiice, an intermediate station, a link circuit terminating at said central office, switching means at said subscribers station for interconnecting said central ofiice and said intermediate office by way of. said link circuit for the reception. at said intermediate ofiice, of calls originating at said central office and means for rendering said link circuit inaccessible at said intermediate ofiice for the origination of calls to said central office. I

4. In a telephone system, a subscribers line, a private branch exchange, a central ofiice line normally connected to saidsubscribers line, a trunk circuit terminating at said private branch exchange, switching means associated with said subscribers line for disconnecting said central ofiice line from said. subscribers line and connecting it to said trunk circuit for directing calls incoming over said central oflice line and intended for said subscribersline to said private branch exchange and means included in said. trunk circuit for rendering said central office line inaccessible to the attendant at said private branch exchange for originating outgoing calls when said switching means is actuated.

5. In a telephone system, a subscribers line, a central office line for connecting said subscribers line with a central ofiice, a private branch exchange, means for directing calls incoming over said central ofiice line from said subscribers line to said private branch exchange and means for rendering said central office line inaccessible from said private branch exchange for initiating calls outgoing over sai'd'central oflice line.

6. In a telephone system, a subscribers line, a. central ofiice line connecting said subscribers line with a central ofiice, a private branch exchange, a trunk circuit terminating at said private branch exchange, a ringing relay associated with said trunk circuit, and switching means associated with said subscribers line for disconnecting, said. subscribers line from said central.

ofiice line and connecting said ringing relay in circuit with said central office line to cause a signal to be displayed at said private branch exchange in response to the actuation of said ringing relay on a call incoming on said central office line, the disconnecting and connecting functions of said switching means being eiiected immediately upon the actuation of said switching means;

'7. In a telephone system, a. subscribers line; a central ofiice line connected to said subscribers line, a private branch' exchange, a link circuit terminating at said private branch exchange, a. ringing relay in said link circuit, switching means associated with said subscribers line for connecting said ringing relay in circuitwith said line and to cause a signal to be displayed at said private branch exchange when said relay is operated on a call incoming on said central ofiice line and means responsive to the seizure of said link circuit at said private branch exchange subsequent to the display of said signal for completing the talking conductors of said .link circuit from said private branch exchange.

8. In a telephone system, a subscribers line, a central ofiice line connected to said subscribers line, a private branchexchange, a link circuit adapted to connect said subscribers line with said private branch exchange, and a pair of signaling devices for said subscribers line, one of said devices permanently associated with said central office line and the other with said link circuit, said devices having different signaling characteristics whereby calls incoming to said subscribers line over said central office line are distinguished from calls originating at said private branch exchange.

9. In a telephone system, anintercepting trunk circuit terminating at a private branch exchange and normally accessible to the attendant at said exchange. for the reception of calls incoming over said trunk, a subscribers line normally associated with said trunk and means responsive to the answer of an incoming call over said trunk by the subscriber on said line for rendering said trunk inaccessible to the attendant at said exchange;

l0.. In a telephone system, a plurality of link circuits, an operators telephone, a key for each of said link circuits, means responsive to. the operationof each of saidkeys for connecting said operator s telephone. with the link circuit corresponding to the actuated key and means joining all of. said keys in a chain circuit whereby cuitincluding all of said keys whereby the operation of. a key nearer the start of said chain circuit automatically disconnects said operators telephone from' the link circuit corresponding to a previously-actuated key and connects it to the-link circuit corresponding to the key located nearer'the start of said chain circiut.

FELIX A. BONOMI. 

